1. Field of the Invention
The present invention relates generally to the cooling of ferrofluid seals. Particularly, the present invention relates to self-cooling ferrofluid seals.
2. Description of the Prior Art
Ferrofluid seals are widely used in industry. Typically, a ferrofluid seal contains a magnetic circuit that is composed of stationary elements such as magnets and pole pieces, and a rotating element such as a shaft. Magnetic fluid is confined between the rotating and stationary elements by the magnetic field of the magnet and formed into a series of liquid O-rings, which provide sealing.
Heat generation has been a common problem for ferrofluid seals that operate at high speed. The viscous heat generated by the magnetic fluid tends to heat up the seal to a level that does not allow for the proper operation of the seal. Typically, water cooling or another coolant is used to overcome this problem. When water is not available, externally introduced forced air convection methods (such as an external fan), or natural convection cooling methods are used.
U.S. Pat. Nos. 4,674,109 and 5,421,892 are examples of liquid cooled ferrofluid seals. U.S. Pat. No. 4,674,109 (1987, Ono) discloses an x-ray tube device with an anode target capable of rotation and a cathode which generates electrons causing them to collide with the target set in a vacuum envelope, and with a shaft which supports and rotates the anode projecting outside the envelope. This x-ray tube device has a structure such that the target is cooled by coolant flowing through coolant channels in the shaft. A vacuum seal is maintained by seal means such as magnetic fluid seal between the envelope and the rotating shaft. The envelope and coolant channels are best maintained at ground potential, and thus have an intermediate potential, with high positive and negative voltages supplied to the anode target and cathode.
U.S. Pat. No. 5,421,892 (1995, Miyagi) discloses a vertical heat treating apparatus that includes a cap body, which is movable up and down, for sealing a treatment vessel that holds objects to be treated. A rotary loading device is provided with a rotary shaft which extends into a through hole provided in the cap body, and a magnetic fluid seal member is provided around the rotary shaft. Heat-exchange media, such as water or ethylene glycol, is circulated within the rotary shaft, preferably to cool the rotary shaft. A temperature sensor may be provided in a housing for the rotary shaft, such that when the temperature exceeds a set temperature, the flow rate of the heat exchange medium is increased. Baffle plates may be provided about an upper surface of the cap body and opposed to the through hole in the cap body. In one embodiment of the invention, nitrogen gas is circulated through the through-hole in the cap body to prevent corrosive gas from contacting the shaft. Circumferential grooves are defined around the rotary shaft at locations where the heat exchange medium is admitted and discharged from the rotary shaft. Preferably, the heat exchange medium is circulated in the rotary shaft above and below the level of the magnetic seal.
The following example uses the Peltier effect to cool a ferrofluid seal. U.S. Pat. No. 5,486,728 (1996, Hirama) discloses a micromotor. The micromotor includes a cylindrical rotor casing having a central through-hole, and a rotor having a cylindrical, magnetic rotor block fixed on a rotor shaft and inserted in the central through-hole of the rotor casing. First and second bearings support the rotor shaft for rotation and are fitted, respectively, in opposite ends of the central through-hole of the rotor casing and define a sealed rotor chamber therebetween. Stator coils are attached to the outer rotor circumference of the rotor casing, and a stator casing is joined to the rotor casing coaxially with the rotor so as to cover the stator coils. A magnetic fluid is filled in the sealed rotor chamber between the first and second bearings between which is disposed the magnetic rotor block. A series of Peltier elements are attached to the outer circumference of the stator casing and electrically connected to a power supply to adsorb heat generated by the operation of the components of the micromotor.
Each of the listed methods has limitations. The natural convection cooling method is frequently unable to provide enough cooling effect, and the seal has a tendency to overheat at high speeds. The externally introduced forced air convection method requires additional space and parts to integrate the fan, which introduces design problems and higher costs. Further, the internal components are difficult to be cooled by this method. When water or other liquid coolant is used, there is always the concern that the liquid coolant may leak out of the cooling channels and cause equipment damage and process contamination. The use of Peltier devices adds additional cost, space and parts to integrate these Peltier devices and further requires power to be supplied to the Peltier devices to effect cooling.
Therefore, what is needed is a cooling system for ferrofluid seals that eliminates coolant leaks. What is also needed is a cooling system that generates effective heat flow path and heat dissipation surface for both the stationary and rotating elements of the ferrofluid seal. What is further needed is a cooling system that generates effective airflow paths within the ferrofluid seal so that both its stationary and rotating elements can be cooled by forced convection. What is still further needed is a cooling system that generates airflow inside the seal to provide effective cooling to both the stationary and rotating elements of the seal. What is yet further needed is a cooling system that provides cooling simultaneously when the seal is operated and where the cooling effect increases proportionally with the operating speed of the seal.